Solid fuel heating appliances

ABSTRACT

Solid fuel heating appliance having high thermal efficiency and low levels of polluting emissions. The heating appliance includes an insulated secondary combustion chamber where a mixture of exhaust gases from the primary combustion chamber and secondary air is burned. A catalytic igniter is provided in the secondary combustion chamber to lower the ignition temperature of the unburned volatile gases from the primary combustion chamber. Regenerative feedback structure is provided in heat exchange relationship with the exhaust from the secondary combustion chamber to preheat the mixture entering the secondary combustion chamber, catalytic igniter, and the regenerative heat transfer assure substantially continuous combustion within the secondary combustion chamber even when conditions in the primary combustion chamber change. In a retrofit embodiment for existing stoves, a sheet metal partition in the secondary combustion chamber allows heat transfer from spent gases to the entering mixture of exhaust and secondary air. In a unitary stove embodiment, secondary air is heated by the spent gases which proceed to remote heat exchangers separated from the primary firebox by ventilated air spaces. To prevent room emissions, the stove door is sealed by a vented double gasket system.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of U.S. Ser. No. 555,511,filed on Nov. 28, 1983, now U.S. Pat. No. 4,510,918, issued Apr. 16,1985, and U.S. Ser. No. 572,000, filed on Jan. 19, 1984, now U.S. Pat.No. 4,582,044, issued Apr. 15, 1986.

This invention relates to solid fuel heating appliances and moreparticularly to such appliances having high heating efficiency whilegenerating low levels of polluting emissions.

As wood burns in a modern, air tight woodburning stove, products of bothcomplete and incomplete combustion are created containing pollutingemissions including particulate material and unburned volatiles whichare discharged into the atmosphere and heavier compounds such ascreosote which can condense onto the inside surface of the chimney flue.The problem is exacerbated when burning at low heat levels in anoxygen-starved mode. Creosote build up is dangerous in that it canignite causing a hazardous chimney fire. The particulate emissions aredamaging to the environment. Not only do the unburned volatiles have adetrimental impact on the environment, but also the heating value ofthese unburned volatiles is wasted as the volatiles are discharged intothe atmosphere.

In an effort to make a cleaner burning stove with higher thermalefficiency, manufacturers have made stoves employing various techniquesfor more complete combustion such as secondary combustion chambers andcatalytic combustors. Known catalytic combustors usually include athick, perforate honeycomb structure of ceramic or other material coatedwith a catalyst material such as platinum, palladium or rhodium. Thesurface properties of these catalyst materials are such that thecombustion products, too cool to burn unaided, will burn within thecatalytic combustor. The conventionally known catalytically equippedstoves are so designed that virtually all of the combustion occurringbeyond the primary firebox volume occurs within the volume of thecatalytic element itself. Combustion ceases downstream of the catalyticelement primarily because the region beyond the catalytic element istypically made of a heat conductive material allowing heat to escapethereby quenching further combustion. Since combustion in known stoveswith catalytic combustors takes place entirely within the volume of thecombustor, these combusters are quite thick. If combustion is notcomplete by the time the gases have exited the combustor, it is unlikelythat any additional combustion will occur. Hence, the thicker, thebetter was the prevailing rule. However, even though the combustor isperforated, its thickness constitutes a significant flow restrictionwhich increases back pressure.

Solid fuel stoves are also know which employ a secondary combustionchamber for further burning of gases from the primary combustionchamber. Generally, however, woodburning stoves with secondarycombustion chambers, even if they are capable of sustaining combustionprior to a log shift, may "wink out" during the exhaust gas compositionchange due to a shift in the fuel load, for example, by a falling log.Even if the exhaust gases return to the same composition shortly afterthe distrubing event, the secondary system may not re-ignite if it hascooled down sufficiently in the meantime. In order to maintain secondarycombustion and a clean burn in a stove with a conventional secondarycombustion chamber, the combination of sensible heat (the temperature ofthe gases before they enter the secondary chamber) and latent heat (theheat released when the combustible constituents are burned in thesecondary) contained in the gas mixture must be high enough to maintaincontinuous temperatures in the secondary chamber above 1000°-1200° F. Ifthe gas mixture changes temporarily such that the total amount of heat(sensible and latent) available to the secondary chamber is insufficientto maintain the proper chamber temperatures, secondary combustion willcease. The gases will not re-ignite no matter how rich until they areagain brought up to a temperature of 1000°-1200° F. when entering thesecondary chamber. In general, re-ignition requires operator attentionsimilar to that required during the initial lighting of the secondarychamber. Operation of a stove with a secondary combustion chamber withthe secondary combustion extinguished is to be avoided since creosoteand other emissions are worse than with a conventional wood stove havingno secondary chamber.

A further problem of conventional secondary combustion chambers involvesheat transfer to the primary chamber. While heat transfer to the room isdesired for thermal efficiency, secondary heat can undesirably influenceprimary combustion. If too much heat from secondary combustion is fedback to the primary combustion chamber, the result can be uncontrolleddevolatization. This interaction interferes with the ability to controlprimary combustion solely by adjusting the primary air.

In an effort to make cleaner burning solid fuel stoves, manufacturershave introduced retrofit units for existing stoves including catalyticcombustors intended to reduce the levels of smoke and creosote andincrease efficiency. Generally, the operation of known retrofit units isunpredictable at best, depending upon the base appliance to which it isattached. This marginal situation is the result of the retrofitcatalytic combustor being located too far from the wood stove firebox,resulting in exhaust gases entering the catalyst at a temperature toolow for optimum catalyst performance, especially when the stove isoperated at lower heat output levels. During low heat output operationwith known systems, the gases exiting the stove body are often at toolow a temperature for sustained catalytic ignition. In such a situation,the catalytic combustor will have little, if any, effect on the levelsof undesirable effluents. Furthermore, because known catalytic elementsare on the order of three inches thick, their use results in elevatedback pressures thereby diminishing draft and resulting in less efficientoperation.

A further problem with highly efficient woodburning stoves is theirincreased tendency toward leakage of light hydrocarbons through thegasketing material. The unavailablity of better sealing asbestosgasketing makes the problem even worse.

It is therefore an object of this invention to provide solid fuelheating apparatus having high thermal efficiency and low levels ofpolluting emissions.

Yet another object of the invention is a solid fuel heating appliance inwhich secondary combustion is maintained during periods of exhaust gascomposition fluctuations.

Yet another object of the invention is an exterior retrofit system forattachment to existing woodburning stoves for reducing harmful emissionsand improving combustion efficiency.

SUMMARY OF THE INVENTION

The applicants herein have discovered that the combination of threeelements, igniter means, preferably catalytic, insulated secondarycombustion chamber and regenerative heat feedback, results in a solidfuel heating appliance in which the secondary combustion chambersustains combustion during and after composition and temperature changesin the exhaust gases from the primary combustion chamber due to shiftsin the solid fuel load, such as the falling of a log when wood is thefuel source. The solid fuel heating appliance disclosed herein includesan insulated secondary combustion chamber, preferably insulated by meansof refractory materials. Preferably, a thin catalytic igniter element isplaced at the entrance to the secondary combustion chamber. Thecatalytic element serves to lower ignition temperature of the primaryexhaust gases to as low as 600° F. A mixture of the primary exhaustgases and secondary air exceeding this temperature and passing throughthe catalytic igniter will be ignited and will continue to burn in theinsulated secondary combustion chamber since the heat of secondarycombustion is conserved in the insulated secondary chamber. Becausesecondary combustion occurs throughout the secondary combustion chamberrather than merely within the confines of the catalytic element itself,more complete combustion is achieved for higher thermal outputefficiency and lower emissions. Moreover, the thickness of the catalyticelement is significantly reduced.

One more element is required to insure sustained combustion within thesecondary chamber. During low heat output operation of a wood stove,either by design or as a result of a shift in the fuel supply, theexhaust gases exiting the stove body are often too low, in the range of350°-500° F., for catalytic ignition by the catalytic igniter.Applicants have overcome this shortcoming by utilizing in a regenerativefashion the heat released in the secondary combustion chamber forpreheating the mixture of secondary air and primary exhaust gases beforethey reach the catalytic igniter to a level at which the gases willignite and burn in the secondary combustion chamber. The applicantsherein have combined the elements of catalytic igniter, insulatedsecondary combustion chamber, and a preheating of the gases entering theigniter to produce both a free-standing unitary stove and a retrofitappliance resulting in higher thermal efficiencies and lower harmfuleffluents.

In addition to the above-mentioned aspects of this invention, in oneembodiment of the invention, a unitary stove, the crossfire primarycombustion system is arranged to force the combustible gases formedduring devolatilization of the wood to pass through the charcoal portionof the fuel bed just prior to exiting the primary chamber. This finalpreconditioning of the exhaust combustibles is important for tworeasons. First of all, the gases are elevated in temperature even at lowfuel comsumption rates because the charcoal bed becomes extremely hot asit consumes any excess oxygen left in the primary gases. Secondly,removing or stripping the excess oxygen from the primary gases removesthe oxygen level variable from the system. Thus, the ability to meterthe proper amount of secondary air into the combustible gases ariseswhen the combustible gases are consistently oxygen depleted rather thancontaining variable levels of oxygen. A secondary air metering device isadded in one embodiment to take advantage of this property. Ideally, anair shutter is controlled as a function of the combustion temperature inthe secondary combustion region.

In this unitary stove embodiment of the present invention, the solidfuel heating apparatus includes a primary combustion chamber for burninga supply of solid fuel such as wood contained therein and a secondarycombustion chamber in gaseous communication with the primary combustionchamber. The secondary combustion chamber is lined with a refractoryinsulating material and includes a perforate catalytic igniter throughwhich combustion gases from the primary combustion chamber flow. Thesecondary combustion chamber further includes insulating refractorybaffles arranged to enhance mixing of the combustion gases and locatedto re-radiate heat onto the catalytic igniter. Manifolds are providedfor introducing secondary combustion air into the secondary combustionchamber so that the combustion gases are more completely burned toimprove heating efficiency and to reduce harmful emissions. The tortuouspath through the secondary combustion chamber caused by the refractorybaffles helps insure more complete combustion because of longerresidence time within the secondary combustion chamber.

In this unitary stove embodiment the combustion gas/secondary airmixture is preheated to insure ignition by the catalytic igniter.Preheating is accomplished by placing the secondary combustion airmanifolds in heat exchange relation with the combustion gases after theyhave passed through the catalytic igniter and have burned in thesecondary combustion chamber. The catalytic igniter has a thickness andperforate open area to minimize a pressure drop across the igniter forimproved draft of the heating apparatus. Not only do the final exhaustgases preheat the gas/air mixture before entering the secondarycombustion chamber, but the exhaust gases also serve to improve thedelivery of heat into a room through side heat exchangers separated fromthe primary and secondary combustion chambers by convective ventilatedair spaces. These side heat exchangers include circuitous passageways toenhance the heat exchange surface area. The separation of the side heatexchangers from the primary fuel firebox (magazine) by ventilated airspace is important to the performance of the stove. While heat must betransferred away from the final exhaust gases to obtain high levels ofthermal performance, the heat from the final exhaust gases must be keptfrom elevating temperatures in the primary firebox which would causeuncontrollable devolatilization of the fuel. The applicants havediscovered that having a side heat exchanger sharing a common wall withthe primary firebox would often cause this uncontrolled devolatilizationprocess which interfered with the ability to control primary combustionby controlling primary air. Separating the heat exchanger from theprimary firebox with a convective air space enhances controlleddevolatilization and also enhances heat transfer to the room.

Stoves which rely on the devolatilization (gasification) of wood in aprimary combustion zone with subsequent combustion of the volatile gasesin a secondary combustion zone often suffer from odor problems due toeven minute quantities of noxious gases escaping the devolatilizationchamber. In another aspect of the invention in a unitary stove having aremovable section, such as a loading door, a double gasket systemprovides for a tight inner seal similar to that in a conventional singlegasket system to prevent migration of most of the gases outwardly fromthe primary combustion chamber. However, regardless of the integrity ofthe seal, small amounts of gases will find their way past. The presentinvention solves this problem by adding a second gasket to form apassageway between the two gaskets. This passage is ventilated with asmall amount of fresh room air and is in direct communication with thefinal exhaust exit. In this way, the small quantities of noxiouscompounds residing in the space between the gaskets are carried up theexhaust stack along with the small amounts of fresh room air and thusnever find their way into the room to cause odor problems.

Another embodiment of the invention is a self-contained retrofit unitincluding the combination of a catalyic igniter and an insulatedsecondary combustion chamber and further including means for preheatinggases entering the secondary combustion chamber utilizing heat generatedthrough combustion in the secondary combustion chamber. The retrofitcombustion system for attachment to a solid fuel heating appliance asdisclosed herein includes a firebox attached to the solid fuel heatingappliance in communication with the flue gases from the heatingappliance. The firebox includes refractory lined walls separated by aheat exchange barrier creating first and second passageways. A perforatecatalytic igniter located at the lower end of the heat exchange barrierallows communication between the first and second passageways. Therefractory lined walls create a secondary combustion chamber for burningthe effluents from the stove body thereby resulting in a cleaner burningoperation. In this embodiment it is preferred that the heat exchangebarrier have a zig zag configuration and be made of stainless steel. Itis also preferred that the refractory lined walls have an undulatingconfiguration to increase residence time and mixing within the secondarycombustion chamber for more complete burning. Secondary air isintroduced into the retrofit unit both before and after the catalyticigniter to insure an adequate supply of oxygen for complete combustion.The heat from combustion in the secondary combustion chamber beyond theigniter is transferred through the heat exchange barrier to heat thegas/secondary air mixture on the other side of the barrier. In this way,secondary combustion is maintained for highly efficient, clean burning.

BRIEF DESCRIPTION OF THE DRAWING

The invention disclosed herein will be understood better with referenceto the drawing of which:

FIG. 1 is a perspective view, partially cut away, of a free-standingsolid fuel heating appliance disclosed herein;

FIG. 2 is a cross-sectional side view taken along section lines 2--2 ofFIG. 1;

FIG. 3 is a top cross-sectional view along section lines 3--3 of FIG. 1;

FIG. 4 is a cross-sectional view of the stove of FIG. 1;

FIG. 5 is a top cross-sectional view of the stove of FIG. 4;

FIG. 6 is a cross-sectional view along the lines A--A of FIG. 5;

FIG. 7 is a cross-sectional view along section lines E--E of FIG. 5;

FIG. 8 is a perspective view of the retrofit apparatus disclosed hereinattached to a solid fuel heating appliance;

FIG. 9 is a cross-sectional view of the retrofit apparatus of FIG. 8;

FIG. 10 is a schematic plan view of the secondary air control componentsshowing the bimetallic coil and air control plate;

FIG. 11 is a side view of the assembled secondary air control device ofFIG. 10 mounted in the wall of the secondary combuston chamber; and

FIG. 12 is a graph relating secondary combustion zone temperature to thesecondary air control plate position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A free-standing version of applicants' invention incorporating thefeatures discussed above is shown in FIGS. 1-7. With reference first toFIGS. 1 and 2, a solid fuel heating apparatus 10 includes a primarycombustion chamber 12 suitable for holding wood (not shown) for burning.Other solid fuels such as coal may also be utilized. As solid fuel burnsin the primary combustion chamber 12, the combustion gases flow througha passageway 14 in the direction shown by arrows 16. The passageway 14is created by an arch 18 and a ramp 20, both made of an insulatingrefractory material. The incline of the ramp 20 aids in turning the gasflow upwardly and also helps prevent ash build-up thereon. As best shownin FIGS. 2 and 4, the passageway 14 leads to a secondary combustionchamber 22 created by a front refractory member 24 and a rear refractorymember 26. The front refractory member 24 is located adjacent to a metalfireback 27 which faces into the primary combustion chamber 12. Thefireback 27 and the refractory arch 18 include ribe 28 which projectinto the primary combustion chamber 12 to maintain an appropriate airspace behind the wood in the primary combustion chamber 12.

The front and back refractory members 24 and 26 are preferablyvacu-formed/fired low density refractory materials. As can be seen inFIG. 2, the members 24 and 26 include integrally formed baffles 32 whichextend into the secondary combustion chamber 22. The members 24 and 26are also adapted to support a catalytic igniter 34. As can be seen inFIGS. 1, 5 and 6, the catalytic igniter 34 is a rectangular honeycombstructure made of a ceramic or metal coated with a catalyst materialsuch as platinum, palladium or rhodium. In the present embodiment, thecatalytic igniter 34 has dimensions of approximately 21/2 inches deep,12 inches long and 1 inch thick. To facilitate combustion in thesecondary combustion chamber 22, secondary combustion air from asecondary air manifold 36 enters the secondary combustion chamber 22through a lower row of openings 38 and an upper row of openings 40, FIG.7. The manifold 36 is well insulated to maintain the secondarycombustion air in a preheated condition as will be discussed hereinbelow.

The introduction of both primary and secondary combustion air will nowbe discussed. With reference first to FIG. 4, primary air enters theapparatus 10 through primary air inlet 31 into a primary manifold 33through which primary combustion air passes into the primary combustionchamber 12. Secondary combustion air enters the apparatus 10 through asecondary inlet 35 entring a secondary manifold 37. With reference toFIGS. 1, 5 and 7, secondary air entering at the secondary inlet 35travels outwardly in the manifold 37 and upwardly through secondary heatexchanger passages 48. From there the secondary air travels downwardlyand then through the holes 38 and 40 into the secondary combustionchamber 22. As will be described below, the heat exchanger passages 48are washed on their outer surfaces by the final exhaust gases from theapparatus 10. In this way, the secondary combustion air is preheatedbefore it enters the secondary combustion chamber 22. With reference nowto FIGS. 1, 2 and 6, the top of the secondary combustion chamber 22 isclosed by a top refractory member 42 which forces the gases from thesecondary combustion chamber toward the sides of the heating apparatus10 and down along the outer surfaces of the secondary combustionchamber. The flow along these surfaces helps maintain a high temperaturein the second combustion chamber. As shown in FIGS. 1, 5 and 6, thegases from the secondary combustion chamber 22 are caused to follow acircuitous path (shown by the arrow 44) by metal barriers 46. As can beseen most clearly in FIGS. 1 and 5, final exhaust gases from thesecondary combustion chamber 22 traveling along the arrow 44 wash pastthe secondary air heat exchanger 48. As shown in FIG. 7, after passingthrough the heat exchanger passages 48, the then preheated secondarycombustion air passes through holes 38 and 40 into the secondarycombustion chamber 22. Thus, outside air is drawn into the secondarymanifold 37 through inlet 35 and passes through the heat exchanger 48where it is preheated by the action of the exhaust gases traveling alongthe arrow 44 and then enters the manifold 36 for delivery to thesecondary combustion chamber.

With reference now to FIGS. 1, 3 and 5, there is shown a convective airspace 50 which separates the heat exchange and exhaust pathways from themain stove body 52. Thus, the final exhaust gases passing along thearrow 44 not only transfer heat to the secondary air within thesecondary air heat exchanger 48, but also cause heat to be transferredto air in the convective air space 50 for delivery into a room to beheated. By separating the heat exchange section from the primarycombustion chamber, the primary combustion chamber is prevented frombecomming overheated which would cause uncontrolled devolatilization ofthe combustion gases.

Another aspect of the present invention will now be discussed withreference to FIG. 4. The top of the primary combustion chamber 12 isclosed by means of a top or griddle 60. The top 60 seals the primarycombustion chamber 12 by means of inner and outer gaskets 62 and 64. Apassageway 66 is created between the gaskets 62 and 64. The passageway66 is in direct communication with the final exhaust exit 68 by means ofa conduit 70. A small opening 72 is provided to allow fresh air to enterthe passageway 66. Because of flowing gases in the exhaust exit 68, anynoxious gases and a small quantity of fresh room air will be drawnthrough the conduit 70 thereby preventing noxious gases from seepingthrough the outer gasket 64 into a room.

The operation of the heating apparatus 10 will now be discussed withreference to FIGS. 1-7. As wood or other solid fuels are burned in theprimary combustion chamber 12, combustion gases are forced to flowthrough the passageway 14 into the secondary combustion chamber 22. Itshould be noted that the passageway 14 is at the lower portion of theprimary combustion chamber 12 and, in particular, is in the region wherea charcoal bed will be formed. Thus, the combustible gases in theprimary combustion chamber 12 formed during devolatilization of the woodpass through the charcoal bed portion of the fuel bed just prior toexiting into the secondary combustion chamber 22. As discussed above,this final preconditioning of the exhaust combustibles both elevates thetemperature of the combustibles and removes excess oxygen from theprimary exhaust gases. The baffles 32 create turbulence to enhancemixing of the combustion gases with secondary combustion air enteringthe secondary combustion chamber 22 through the openings 38 and 40 inthe refractory member 26. The mixture of combustion gases and secondaryair proceeds through the perforate catalytic igniter 34. The catalyticigniter 34 has the property of reducing the temperature at which thecombustion gases/secondary air combination will ignite to aproximately600° F. Thus, as the combustion gas/secondary air combination passesthrough the catalytic igniter 34, combustion is initiated and continuesin the refractory lines secondary combustion chamber 22. The heat ofcombustion combined with the insulating property of the refractorymembers 24 and 26 combine to maintain a high temperature in thesecondary combustion chamber 22. Not only do the baffles 32 enhancemixing by causing turbulence, they also are located to re-radiate heatback onto the catalytic igniter 34 to enhance its performance. Asdiscussed above, the exhaust gases follow a circuitous, heattransferring path both for transferring heat into the room by means ofthe convective air space 50 and also for preheating the secondarycombustion air.

The features of the present invention--namely, the combination of acatalytic igniter/insulated secondary combustion chamber along withregenerative preheating--have also been embodied in a self-containedretrofit unit adapted for connection to existing solid fuel burningstoves. With reference now to FIG. 8, an external retrofit apparatus 110is shown affixed to a solid fuel heating applicance 112, such as aVigilant® Wood Stove manufactured by Vermont Castings, Inc. The exteriorretrofit apparatus 110 includes an attachment portion 114 which isadapted to be bolted directly onto the stove 112 in place of the stove'soriginal flue collar (not shown). A flue collar 116 is then bolted ontothe exterior retrofit apparatus 110. The height of the flue collar 116remains the same as it had been on the wood stove 112 and horizontal andvertical flue position options are retained. In general, the onlymodifications necessary for the installation of the retrofit unit 110 isrepositioning the stove 112 forward approximately 6 inches. The retrofitunit 110 is approximately 14 inches wide, 61/2 inches deep and 18 incheshigh. It is preferred that the external components of the unit 110 bemade of cast iron or cast iron in combination with sheet metal oraluminum. The retrofit unit 110 will now be described in detail withreference to FIG. 9. The retrofit unit 110 is attached to thewoodburning stove 112 so that exhaust gases from the stove 112 enter theexterior retrofit apparatus 110 as indicated by an arrow 120. Theretrofit apparatus 110 is divided front to back by a stainless steelheat exchanger 122 forming a first passageway 124 and a secondpassageway 126. As shown, the heat exchanger 122 has a zig-zag orundulating shape to increase surface area for better heat exchange. Thewalls of the retrofit apparatus 110 are lined with refractory material128 which also has an undulating shape to increase the effectivecombustion chamber length and therefore to increase the residence timeof the gases. The refractory material 128 is preferably vacuum-formed,insulating refractory material. The undulating shape of the refractorymaterial 128 also improves mixing for more efficient operation. Openings129 are provided to allow secondary air to enter the retrofit unit 110both before and after a catalytic igniter 130. The catalytic igniter 130is disposed in the lower portion of the retrofit apparatus 110. Thecatalytic igniter 130 is made of a honeycomb ceramic substrate coatedwith a catalyst such as platinum. Other catalysts and substrates mayalso be appropriate. The catalytic igniter 130 is approximately 1 inchthick. The relative thinness of the catalytic igniter 130 minimizes thepressure drop across the igniter 130.

The operation of the retrofit apparatus 110 will now be discussed.Products of combustion from the wood stove 112 enter the retrofitapparatus 110 along the arrow 120 and flow downwardly through the firstpassageway 124 and subsequently through the perforate catalytic igniter130 into the passageway 126. As the gases pass through the catalyticigniter 130, they are ignited and burn further in the secondarycombustion area indicated by the bracket 132. Secondary combustion airenters the unit 110 through the openings 129 to provide an adequatesupply of oxygen for complete combustion. The catalytic igniter 130 isan igniter with a substantial portion of the combustion occurringoutside the confines of the catalytic igniter 130 itself. The productsof the secondary combustion travel upwardly through the passageway 126and exit through a flue collar 134. As the gases travel upwardly in thepassageway 126, they pass across the heat exchanger 122 transferringheat into the passageway 124 since gases in the passageway 126 aresubstantially hotter than those in the first passageway 124 which havenot yet been secondarily burned.

The internal heat exchange capability of the apparatus 110 is animportant aspect of this invention. For example, during low heat outputoperation of the appliance 112, exhaust gases exiting from the appliance112 are often in the temperature range of 350°-500° F. which may be toolow for catalytic ignition by the catalytic igniter 130. By mean of heattransfer through the heat exchange panel 122 the gases are preheated toa temperature of 500°-650° F. or higher which is sufficient forsustaining catalytic ignition and subsequent secondary combustion in theretrofit apparatus 110. Also, a cleaner burn will result at higher heatoutputs even when the temperature of the gases entering the catalyticigniter is already high enough for catalytic ignition. By alwaystransferring sensible heat to the gas stream entering the catalyticigniter/insulated secondary combustion chamber from the relativelyhotter final exhaust, the highest possible temperatures are maintainedin the secondary combustion area 132 for more nearly complete burning ofthe gases. A result of the use of the retrofit apparatus 110 is higherstack temperatures at low heat output than would be the case with atypical non-catalytic stove burning at a comparable pound per hour rateof fuel consumption. The resulting higher stack temperatures of theretrofit apparatus 110 at low heat output can help prevent creosotecondensation within the stove pipe or chimney and also improve low draftproblems in installations having marginal draft and/or during warmweather.

Still referring to FIG. 9, a damper 136 is provided which is integralwith the retrofit apparatus 110 and which directs gases down through thepassageway 124 when it is in the position illustrated in FIG. 9 anddirectly through the flue collar 134 when it is lowered into theposition shown in phantom. The lowered position is utilized duringloading of wood into the heating appliance 112 or during start up.

The above-described retrofit apparatus 110 is designed for cleanoperation in the heat output range of from 20,000-50,000 BTU's per houror approximately 4-10 pounds of wood per hour. Within this range, thereshould be a significant reduction in smoke and creosote emitted from theflue. The combination of the refractory lined secondary combustionchamber and catalytic igniter along with regenerative preheating resultsin continued secondary combustion even if ideal conditions are notmaintained. Thus, even if heat output drops, secondary combustion willcontinue without any operator attention. This characteristic isimportant because stoves are often operated for long periods of timewithout any attention. The insulated refractory lined secondary chamberprovides the gases with the residence time at elevated temperaturesnecessary for more complete combustion.

The performance of both the unitary stove and retrofit embodiments isfurther enhanced by the addition of a secondary air control device.

As shown in FIGS. 10 and 11, one embodiment of this device is comprisedof a high temperature heat conducting rod 200, a bi-metallic thermostatcoil 202, a wire connecting linkage 204 and a specially shaped aircontrol plate 206 pivotally mounted at 206a over the secondary air inletorifice 208.

This device senses the temperature within the secondary combustion zone210 (FIG. 11) and then regulates (or meters) the secondary air flow insuch a way as to maximize the temperature within the secondarycombusiton zone.

This is accomplished by the use of the high temperature heat conductingrod 200 inserted through the seconary combustion chamber wall 212 withinthe desired region within the secondary combustion zone 210. Rod 200transfers heat to the externally mounted bi-metallic thermostat coil 202in a manner proportional to the secondary combustion zone temperature.The bi-metallic coil 202 reacts to the rod temperature and causes motionof the connecting linkage 204 and finally the air control plate 206. Theangular position of the air control plate over the secondary air inletorifice determines the secondary air flow.

The shape of the air control plate 206, the shape of the secondary airinlet orifice 208, the length of the connecting linkage 204, thecharacteristics of the bi-metallic coil 202 and the length and locationof rod 200 can be varied to obtain the desired control characteristics.

In the preferred embodiment, the secondary air orifice remainsessentially closed until temperatures within the secondary combustionzone exceed 1200° F. Air is introduced as the rod senses increasingsecondary zone temperatures and the air control orifice is essentiallyfully open when the secondary zone has reached 1700° F. The amount ofair introduced once the control senses that air is required is in arelationship proportional to the secondary combustion zone temperatureover the 1200°-1700° F. range. The proportionality may be a simplelinear relationship or be a more complex geometric or quadraticrelationship. FIG. 12 shows the envelope which represents the optimumrange of secondary air versus secondary combustion zone temperature.

At temperatures less than 1200° F., the introduction of additional airto the secondary combustion zone is often a liability. The air can causea "quenching" effect for two reasons. First, it can lower thetemperature within the secondary zone and secondly it can dilute thecombustible gas mixture. Both of these effects will decrease theprobability of ignition of the combustibles within the secondary zone.

The rate at which secondary air should be introduced in the 1200°F.-1700° F. range depends in part on the design of the secondarycombustion system. A simple linear relationship between air flow andsecondary temperature may be adequate for one design while a morecomplex relationship may give better results for a different combustionsystem. Variations on the illustrated design provide a variety of aircontrol relationships.

It is thus seen that the objects of this invention have been achieved inthat there has been disclosed solid fuel heating apparatus capable ofhigh thermal efficiency and low levels of polluting emissions. Theheating appliance as disclosed herein achieves these results by means ofan insulated secondary combustion chamber where combustion is sustainedafter ignition by a catalytic igniter. Importantly, the combustiblemixture entering the catalytic igniter is preheated in a regenerativefashion by means of heat from the final exhaust gases. As describedabove, this arrangement provides for sustained secondary combustionresulting in more heat being extracted from the solid fuel source andalso resulting in cleaner waste products without adversely affectingprimary combustion. In particular, the thin catalytic element utilizesthe catalytic property in the most appropriate way, namely as anigniter, not a combustor. This feature enables the catalytic element tohave a low profile reducing its undesirable flow restrictive properties.The unitary stove embodiment maintains control over primary combustionby isolating the final exhaust heat exchangers and by oxygen-depletingthe primary exhaust so that primary and secondary combustion can beindependently controlled.

It is recognized that modifications and variations of theabove-described embodiment will occur to those skilled in the artwithout departing from scope or principles of the invention. Forexample, features of the retrofit embodiment can be employed in theunitary stove and vice versa. Moreover, while a catalytic-type elementis preferred as the igniter means, other devices may be usefullyemployed in the entrance to the secondary combustion region to achieveessentially the same effect of lowering the required temperature of theincoming primary exhaust gases necessary to ultimately achieve ignitionand combustion in the secondary combustion chamber. It is intended thatall such modifications and variations be included within the scope ofthe appended claims.

What is claimed is:
 1. A solid fuel burning heating stove with enhancedsecondary combustion comprising:a primary combustion chamber for burninga load of fuel contained therein with a primary outlet for primaryexhaust laden with unburned volatiles; an insulated secondary combustionchamber having an entrance followed by a combustion region wheresecondary combustion predominates and a final exhaust region of spentgases; a passageway leading from said primary outlet to said entrance;means for admitting secondary air to said passageway and for producing amixture of secondary air and primary exhaust before said entrance;igniter means spanning said entrance for encouraging ignition of saidsecondary air/primary exhaust mixture by lowering the temperature of themixture required to achieve ignition; means for admitting additionalsecondary air to said secondary combustion region after said ignitermeans; and regenerative feedback means in heat exchange relationshipwith said final exhaust for preheating said mixture before encounteringsaid igniter means using the sensible heat of said spent gases; wherebysecondary combustion is sustanied by the availability of the ignitermeans to overcome fluctuations in the temperature and composition of theprimary exhaust which might otherwise foil ignition, and by using theheat of spent gases rather than primary combustion to maintain thetemperature of the mixture entering the secondary combustion chamber atan elevated level.
 2. The apparatus of claim 1, wherein saidregenerative feedback means is formed by a common wall between saidfinal exhaust region and said passageway acting as a heat exchanger. 3.The apparatus of claim 2, wherein said common wall is made of thermallyconductive material.
 4. The apparatus of claim 2, wherein said commonwall is stainless steel.
 5. The apparatus of claim 2, wherein saidcommon wall has a convoluted configuration to present a large surfacearea to promote heat transfer.
 6. The apparatus of claim 1, wherein saidregenerative feedback means includes means for preheating the secondaryair admitted to said passageway with the heat of said spent gases tothereby preheat said mixture.
 7. The apparatus of claim 6, furthercomprising means for preheating the secondary air admitted to saidsecondary combustion region with the heat of said spent gases.
 8. Theapparatus of claim 1, further comprising heat exchanger meanscommunicating with said secondary combustion chamber separated from saidprimary combustion chamber by a room air space for transferring the heatof the spent gases to the room air,whereby heat transfer to room air isaccomplished without interfering with primary combustion.
 9. Theapparatus of claim 1 wherein said primary combustion chamber includesmeans for supporting a bed of coals, and means for forcing the primaryexhaust through said bed of coals just before exiting to said passagewayto enhance oxygen depletion of said primary exhaust to maintain aconsistent level of oxygen in said mixture.
 10. The apparatus of claim 9wherein said means for admitting additional secondary air includessecondary air control means for controlling the amount of secondary airadded to said secondary combustion region as a function of temperatureinside said region such that the secondary air volume is adjusted from aminimum to a maximum level over a predetermined temperature range. 11.The apparatus of claim 10 wherein said minimum level has no additionalsecondary air.
 12. The apparatus of claim 10 wherein said range is fromabout 1300° F. to about 1700° F.
 13. The apparatus of claim 1 whereinsaid igniter means is a catalytic igniter.
 14. The apparatus of claim 13wherein said catalytic igniter has a thickness and perforate open areato reduce the pressure drop across the igniter for improved draft. 15.The apparatus of claim 13 wherein said catalytic igniter includes aceramic substrate coated with a catalyst.
 16. The apparatus of claim 13wherein said catalytic igniter includes a metal substrate coated with acatalyst.
 17. The apparatus of claim 1 wherein said secondary combustionchamber is insulated with refractory material.
 18. The apparatus ofclaim 1 wherein said regenerative feedback means comprises secondarycombustion air manifolds in heat exchange relation with said finalexhaust.
 19. The apparatus of claim 1 wherein said stove includes sideheat exchangers separated from the primary and secondary combustionchambers by convective air spaces for enhanced heat transfer into a roomto be heated.
 20. The apparatus of claim 19 wherein said side heatexchangers include circuitous pathways to enhance heat exchange surfacearea.
 21. The apparatus of claim 1 wherein the entrance to saidsecondary combustion chamber includes a lower ramp portion for guidingcombustion gases and for substantially preventing ash build-up in saidsecondary combustion chamber.
 22. The apparatus of claim 1 wherein saidsecondary combustion chamber includes refractory baffles arranged toenhance mixing of the combustion gases and to re-radiate heat onto thecatalytic igniter.
 23. The apparatus of claim 1 wherein said secondarycombustion chamber is lined with insulating material having anundulating surface to enhance mixing.
 24. The apparatus of claim 1wherein said secondary combustion chamber includes a heat transferringbarrier creating first and second passageways, the first passagewayadapted for conveying primary exhaust through said catalytic igniterwhereby secondary combustion heat in said second passageway istransferred to the exhaust gases in the first passageway through theheat transfer barrier.
 25. The apparatus of claim 1 wherein said meansfor admitting additional secondary air includes secondary air controlmeans for controlling the amount of secondary air added to saidsecondary combustion region as a function of temperature inside saidregion such that the secondary air volume is adjusted from a minimum toa maximum level over a predetermined temperature range.
 26. Theapparatus of claim 25 wherein said minimum level has no additionalsecondary air.
 27. The apparatus of claim 25 wherein said range is fromabout 1300° F. to about 1700° F.
 28. A solid fuel burning heating stovewith enhanced secondary combustion, comprising:a primary combustionchamber for burning a load of fuel contained therein with a primaryoutlet for primary exhaust laden with unburned volatiles; a refractorylined secondary combustion chamber having as entrance followed by acombustion region where secondary combustion predominates in a finalexhaust region of spent gases; a passageway leading from said primaryoutlet to said entrance; means for admitting secondary air to saidpassageway and for producing a mixture of secondary air and primaryexhaust before said entrance; catalytic igniter means spanning saidentrance for encouraging ignition of said secondary air/primary exhaustmixture by lowering the ignition temperature of the mixture; means foradmitting additional secondary air to said secondary combustion regionafter said igniter means; and regenerative feedback means in heatexchange relationship with said final exhaust for preheating saidmixture before encountering said catalytic igniter using the sensibleheat of said spent gases, whereby secondary combustion is sustained bythe availability of the igniter means to overcome fluctuations in thetemperature and composition of the primary exhaust which might otherwisefoil ignition, and by using the heat of spent gases rather than primarycombustion to maintain the temperature of the mixture entering thesecondary combustion chamber at an elevated level.
 29. The apparatus ofclaim 28 wherein said primary outlet for primary exhaust is located toforce the primary exhaust through a charcoal bed within the stoveresulting from primary combustion to heat the primary exhaust and tosubstantially deplete the primary exhaust of oxygen.